Reply To: Windows defender turns on automatically

The above answer would only disable the real-time monitoring part of Windows Defender. If you want to disable the service entirely, go here HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\WinDefend and change the value of Start to 4. This will disable the service, while changing the value to 3 will change it to manual start. I hope you are aware of the risks involved in disabling it.